Atomization technology is the breakup of a liquid into small droplets, usually in a high-speed jet or film. The production of high-quality powders, such as aluminum, brass, nickel alloys, cobalt alloys, wear resistant steel, and the like have been produced using the atomization technology. As simply defined, atomization is the breakup of a liquid to form droplets, typically smaller than about 150 .mu.m. The breakup of a liquid stream brought about by the impingement of high-pressure jets of water or gas is referred to as water or gas atomization, respectively. The use of centrifugal force to break up a liquid stream is known as centrifugal atomization; the use of vacuum is known as vacuum atomization and the use of ultrasonic energy to effect breakup of a liquid stream is referred to as ultrasonic atomization. By regulating the parameters of the atomization process, the particle size, particle size distribution, particle shape, chemical composition and microstructure of the particles can be varied.
Conventional water and gas atomization processes presently account for the bulk of atomized metal powders. Water-atomized powders generally are quite irregular in shape and have relatively high surface oxygen contents. Gas-atomized powders, on the other hand, generally are more spherical or rounded in shape and, if atomized by an inert gas, generally have lower oxygen (oxide) contents. The major components of a typical atomization installation include a melting facility, an atomizing chamber, and powder drying (for water atomization) equipment. Melting of metals follows standard procedures. Air, inert gas and vacuum induction melting, arc melting, and fuel heating are suitable procedures.
The molten metal can be poured into a tundish, which is essentially a reservoir that supplies a uniform and controlled flow of molten metal to the tundish nozzle. The nozzle, which can be located at the base of the tundish, controls the shape and size of the metal stream and directs it through an atomizing nozzle system in which the metal stream is disintegrated into fine droplets by the high-velocity atomizing medium. Liquid droplets cool and solidify as they settle to the bottom of the atomization tank. This tank may be purged with an inert gas to minimize or prevent oxidation of the powder. In gas atomization, the powder may be collected as dry particles or cooled with water at the bottom of a tank. In dry collection, the atomization tank could be tall to ensure solidification of the powder particles before they reach the bottom of the collection chamber. Horizontal gas atomization using long horizontal tanks could also be used.
There are various types of gas and water nozzles known in the art to control the parameters of the atomization process to produce a desired powder product.
It is disclosed in the art that typical metal flow rates through single orifice nozzles could range from about 10 to 200 lb/min; typical water flow rates range from 30 to 100 gal/min at water velocities ranging from 230 to 750 ft/s and pressures from 800 to 3000 psi. Typical gas flow rates range from 40 to 1500 scfm at gas pressures in the range of 50 to 1200 psi. Gas velocities depend on nozzle design and may range from 60 ft/s to supersonic velocities. The temperature differential between the melting point of the metal and the temperature at which the molten metal is atomized (superheat of the molten metal) is generally about 75.degree. to 300.degree. C. (135.degree. to 572.degree. F.). There are many other variations to the atomization process known in the art to produce powder products.
U.S. Pat. No. 5,126,104 discloses a method for preparing an intimate mixture of powders of nickel-chromium-boron-silicon alloy, molybdenum metal powder, and Cr.sub.3 C.sub.2 /NiCr alloy suitable for thermal spray coatings which comprises milling a starting mixture of the above two alloys with molybdenum powder to produce a milled mixture wherein the average particle size is less than about 10 micrometers in diameter, forming an aqueous slurry of the resulting milled mixture and a binder which can be an ammoniacal molybdate compound or polyvinyl alcohol, and agglomerating the milled mixture and binder. The intimate mixture and binder may be sintered in a reducing atmosphere at a temperature of about 800.degree. C. to 950.degree. C. for a sufficient time to form a sintered, partially alloyed mixture wherein the bulk density is greater than about 1.2 g/cc. The resulting sintered mixture may be entrained in an inert carrier gas, passed into a plasma flame wherein the plasma gas can be argon or a mixture of argon and hydrogen, and maintained in the plasma flame for a sufficient time to melt essentially all of the powder particles of the sintered mixture to form spherical particles of the melted portion and to further alloy the sintered mixture, and cooled.
U.S. Pat. No. 3,846,084 discloses a composite powder for use in producing articles or coatings having unique wear and frictional characteristics consisting essentially of a chromium matrix with at least one chromium carbide taken from the class of carbides consisting of Cr.sub.23 C.sub.8 ; Cr.sub.7 C.sub.3 ; and Cr.sub.3 C.sub.2 and each particle containing from about 0.2 wt. percent to about 5.4 wt. percent carbon.
U.S. Pat. No. 4,725,508 discloses the use of chromium carbide (Cr.sub.3 C.sub.2) powder for use in thermal spray processes. Many of the chromium carbide powders are produced using the sintering techniques known in the prior art.
Although the atomization process has been known since 1945, it was not appreciated that this process could be used to produce a powder that contained a large volume fraction of chromium carbide phases.
It is an object of this invention to produce an atomized powder of chromium carbide particles dispersed in a nickel chromium matrix.
It is another object of this invention to produce powders using low cost raw materials and minimum process steps.
It is another object of the invention to produce an atomized powder of chromium carbide particles dispersed in a nickel chromium matrix in which the chromium is in an amount in weight percent of the powder from 55 to 91; the nickel in an amount in weight percent of 5 to 40 of the powder; and carbon in an amount in weight percent of 1 to 10 of the powder.